When dewatering cellulosic fibre suspensions, such as in papermaking, it is desirable that as much as possible of the fibres and the additives such as filler, e.g. calcium carbonate, clay, titanium dioxide and the like, be retained in the dewatered product, i.e. the sheet of paper. To this end, it is known to add to the cellulosic fibre suspension (stock) a retention agent which improves the retention, i.e. increases the amount of solid substance, such as filler in the finished paper compared with the case where no retention agent is added.
With a view to improving the retention in the dewatering of cellulosic fibre suspensions, it is disclosed in CA 1,044,782 to use a combination of a phenol-formaldehyde resin and a polyethylene oxide. Moreover, it is disclosed in SE 8604975-6 to improve the retention by adding a phenol-formaldehyde resin and a high-molecular polyethylene oxide in combination with a cationic starch derivative or a cationic cellulose derivative. Finally, it is also disclosed in U.S. Pat. No. 5,516,405 to improve the filler retention in the dewatering of a pulp slurry by adding a cationic fixative, a promoter and a polyethylene oxide. The cationic fixative is selected from the group consisting of polyethylene amides, polydiallyldimethyl ammonium chlorides and polyethylene imines. The promoter can be selected from the group consisting of phenol formaldehyde, naphthalene sulphonate and black liquor.
Even if the retention is improved by using a combination of phenol-formaldehyde resin and polyethylene oxide as retention agent, there is a need of retention agents and methods which result in additionally improved retention. The above-mentioned U.S. Pat. No. 5,516,405 is an example of an attempt at achieving this by using a further additive in the form of a cationic fixative in addition to phenol formaldehyde and polyethylene oxide. Such further additives, however, cause further costs and work, for instance, in determining the correct dosage, dosing occasion and dosing place.
The present invention aims at obviating or decreasing the drawbacks of the prior-art technique and providing a simple agent and method for improving the retention when dewatering a cellulosic fibre suspension.